This invention relates to power amplification of pulse modulated signals by a switching power stage. The invention may advantageously be used for improved conversion of a digital signal directly to analog power. Applications are direct digital audio power conversion and general DC-DC or DC-AC power conversion systems controlled from the digital domain.
The advantages of power amplification based on a switching power stage are well known. The high efficiency provides several advantages, in terms of minimal weight and volume, higher power handling capability and improved reliability. The fundamental elements in switching power amplification are the modulator, the switching power stage and the demodulation filter to reconstitute the modulated signal. The non-linearity of the switching power stage presents a significant impediment to maintain the modulator performance throughout the subsequent power conversion by a switching power stage. This problem is fundamental and independent on the use of analog modulation as analog PWM, or digital modulation as direct digital PCM-PWM conversion.
In prior art, compensation for the non-ideal behavior has generally been attempted with various feedback control methods. U.S. Pat. No. 4,724,396 and U.S. Pat. No. 5,521,549 discloses examples of this method with audio power amplification as specific application. However, linear feedback control methods require an analog low frequency reference. It is desirable with a more direct digital source to power conversion to simplify the power conversion in that no separate D/A converter is needed. Furthermore, analog modulation circuitry and carrier generators will not be required. International patent application no. WO92/11699 and international patent application no. WO97/37433 discloses improved methods for digital PCM-PWM conversion for digital digital to power amplification. In practice, these systems are hardly easy to implement, since compensation for the fundamental error sources within the switching power conversion are not provided. The application of digital feedback control referenced to the digital source is complicated in that an analogxe2x80x94digital converter is needed in the feedback path. This renders normal feedback control impractical.
The published international patent application EP758164 discloses a feedback method local to the switching power stage, in which the power stage output is feed back and summed with a pulse width modulated input. The resulting signal is used to drive the switching power stage. The compensation effect however will be limited and it is difficult to control the improvements towards all error sources.
In conclusion, no invention in prior art exist for general power amplification of a pulse-modulated signal, that incorporates simple and effective means to eliminate any source of non-linearity and noise in the switching power amplification stage.
Accordingly, the primary objective of the present invention is to enable improved power amplification of a pulse modulated signal, where all error sources related to the power stage and demodulation filter are eliminated, such that the modulator performance can be maintained throughout the subsequent elements all the way to the output. Another objective of the invention is to provide a practical digital PCMxe2x80x94power conversion system, that is insensitive to power stage non-linearity, perturbation on the power supply and any other non-ideal elements.
According to claims 1 and 2, the objectives of the invention are achieved by the introduction of a correction unit in-between the pulse modulator and the switching power amplification stage. The correction unit provides compensation by means of pulse re-timing on the pulse edges, said re-timing controlled to have a xe2x80x9cpredistortingxe2x80x9d effect, such that the resulting switching power stage output is free from distortion, noise or any other undesired contribution.
According to claim 3, one preferred embodiment of the invention is particular in that pulse re-retiming in the correction unit is a linear function of an error signal input. This is advantageous in that the resulting control system is linear, whereby system design and optimization is simplified and the performance improvement controllable.
According to claim 4, it is expedient that the pulse reference control system comprises the following fundamental elements:
An input terminal taking a pulse modulated signal.
A Correction Unit with means to correct the delays of the individual pulse edges, controlled by a control input.
A state feedback with compensation.
A reference-shaping block to modify the pulsed reference input for optimal error estimation.
A difference block to generate an error signal and a compensator to shape this error.
The invention includes various embodiments regarding actual realization of the control function within the correction unit. According to claims 6-10, the invention includes various advantageous embodiments concerning improved digitalxe2x80x94analog power conversion suitable for audio power amplification.
The invention is a fundamentally new control method for improved amplification of a pulse modulated input signal. The application range is extremely broad in that the invention can be used with any pulse-modulated input, modulated in the analog or digital domain, and feed any load where a pulsed power signal of controllable quality is needed. Ultimately, the principle of the invention may realize perfect reproduction of the pulsed reference such that the output is a constant times the input, independent on any disturbances that is introduced during power conversion.